The present invention relates to drill head units which are used in conjunction with hollow drill bits which have a brazed carbide cutting tip mounted on the end to drill vertical holes in the ceilings of mine tunnels. To protect against falling ceilings in mine tunnels, vertical holes are drilled into solid strata in the ceiling of mine tunnels into which bolts are inserted. The bolts secure covering material to the underside of the tunnel ceiling for the protection of miners.
To drill a vertical hole into the ceiling of a tunnel, a drill head unit is provided which is connected at one end to a horizontally mounted rotary motor and the top thereof is adapted to receive a vertically mounted drill chuck for holding a hollow drill bit in a vertical position. The drill head unit includes a horizontally mounted bevel pinion gear which is driven by a rotary motor and a vertically oriented bevel gear which drives the drill chuck. A typical existing drill head unit can be seen in U.S. Pat. No. 4,294,317.
Existing drill head units, however, have not been satisfactory for a number of reasons. First, the average life of a drill bit used in such units is often considerably less than would be expected from normal wear of the cutting edges of the bit. In many cases, a drill bit is destroyed after drilling through only five to twenty feet of rock.
A second problem with existing drill head units such as shown in U.S. Pat. No. 4,294,317 is that the same drill head unit cannot be used with both a vacuum system and a water system for removing residue stone dust. Residue dust from the drilling tends to drop around the outside of the drill and cover the top of the drill head unit, and when the stone particles enter into the unit, they cause damage to the parts. Existing drill head units have a drill chuck fitted into splines within a bevel gear. The splines in the gear endure a great deal of wear and fail long before the teeth of the gear fail, however, failure of the splines in the gear results in the loss of the entire gear, which is one of the most expensive parts of the unit.
To reduce the accumulation of stone particles and therefore the damage they cause to the parts, a vacuum or water removal system is provided. When a vacuum is used, the vacuum is drawn down through the hollow drill such that stone dust residue is drawn through the drill and out the bottom of the drill head housing. When a water system is used, pressurized water is forced up through the hollow drill and allowed to run out the bottom of the hole in the ceiling of the mine thereby washing away excess stone particles. Although the vacuum removal system is preferred when it is usable because the stone dust can be collected as a part of the removal process, the vacuum removal system is not practical when the drill is required to pass through very hard rock. A great amount of heat is created while drilling through hard rock and the heat drastically reduces the useful life of the carbide drill bit unless the water removal system is used which also provides a coolant to the drill bit.
Existing drill head units such as shown in U.S. Pat. No. 4,294,317 provide for a complicated spring loaded axial seal which seals the vacuum or water chamber of the drill head unit from the lubricated portions of the drill head unit. When such a drill head unit is constructed to be used with a water removal system, one of the surfaces of the axial seal within the drill unit is made of tungsten carbide which is ground smooth to a millionth of an inch tolerance, and the other sealing surface is made of a ceramic material. Such seals have been found to be very expensive to manufacture, and as a result, when such a drill head unit is constructed to be used with a vacuum removal system, a less expensive carbon material is substituted for the tungsten carbide surface of the seal. The carbon seal is adequate in a vacuum removal system, but parts of such a seal are subject to rust and rapidly fail when a drill head unit fitted with such seals is used with a water removal system. It would therefore be desirable to provide a sealing system for such drill head units which is not excessively expensive and for which the same unit can be used with either a vacuum or water removal system.
Several factors in addition to heat contribute to the shortened life of drill bits used in existing units. As previously mentioned, existing units use splines to convey rotational movement from the vertically mounted bevel gear to the drill chuck and drill shaft. When the splines become worn, the drill chuck cannot be accurately centered in the vertically mounted bevel gear. The shaft of the drill bit used in such units is often four to ten feet in length and if the drill chuck is not centered with respect to the vertical bevel gear, the drill bit will cut a hole with an enlarged diameter and cause the drill and portions of the drill head unit to vibrate as the uncentered drill rotates.
Even when a drill chuck appears to have initially been properly centered in the bevel gear at the outset of drilling, the drill chucks of existing drill head units tend to become uncentered during drilling. When drilling commences, the torque conveyed from the bevel gear to the drill chuck locks the drill chuck in the orientation in which it is positioned at commencement of drilling. As the drill passes through alternately hard and soft strata of rock existing drill head units will chatter and cog during which uneven torque loads are transferred from the motor, through the gears to the drill chuck. As a result of the alternate application and releasing of torque to the drill chuck, the drill chuck will become unseated and reseated into a new uncentered orientation causing the drill bit to wear unevenly and vibrate as described above. Such reseating and the excessive wear caused thereby increases as the splines become subject to wear.
In existing drill units used in mining, the drill steel is slidably received within the drill chuck such that, after a hole has been drilled, and the drill unit is lowered, the drill steel and drill bit will remain stuck in the hole. The operator must then manually grasp the shaft of the drill and withdraw it from the hole. In so doing, the operator consumes several seconds of time which becomes a substantial loss when one considers that the time incurred to drill such a hole takes less than a minute. Furthermore, the operator exposes his hands to possible injury while withdrawing the drill steel.
It would, therefore, be desirable to provide a drill head unit which permits the drill chuck to be self-centering and not subject to realignment as a result of the drill passing through strata of alternately hard and soft material and which would eliminate or reduce chatter and cogging and the damage caused thereby. It would also be desirable to provide a drill head unit which has a relatively inexpensive seal which is usable with both a water removal system and with a vacuum removal system. It would also be desirable to provide a drill head unit which will grasp the drill steel such that the drill steel will be retained within the chuck as the drill head is withdrawn from the hole after drilling.